There has been considerable effort expended in developing tools for analyzing materials. This interest is particularly strong in the integrated circuit industry. In the latter field, there is a need to develop analytical tools which are capable of nondestructively evaluating thickness and compositional variables of thin films with an extreme degree of accuracy. As can be appreciated, knowledge of a film's electric or dielectric properties and its thickness aids in the design and manufacture of highly sophisticated electronic components.
The composition of a material directly effects its ability to transport energy. There are two primary modes of energy transport, namely, electrical and thermal conductance. In metals, electrical and thermal conductivities are directly related because free electrons are the primary mechanism for the transport of both electrical current and thermal energy. This relationship is defined by the Weidemann-Franz-Lorentz model. Thus, at least for metals, if characteristics of one energy transport system is measured, information about the other energy transport system can be mathematically derived. Once the electrical or thermal conductivities are derived, it is possible to obtain data relating to the material's composition or stoichiometry. For dielectric films there is no relationship between electrical and thermal conductivities. Nevertheless, the thermal conductivity will provide information about the composition or stoichiometry of the dielectric material.
Analysis of metallic materials through the electrical transport system is fairly well developed. Typically, the analytical tools are designed to measure resistivity which is related, in an inverse manner, to electrical conductivity. Referring to FIG. 1, there is illustrated a common analytical tool for evaluating a sample based on resistivity. More specifically, a sample 2 is shown connected to a device known as a four-point probe 4. .The four-point probe includes two voltage probes 5 and 6 which are placed in contact with the surface of the thin film layer 3 of the sample. A voltage is placed across the probes through a power source 7. A second pair of probes 8 and 9 are placed in contact with the layer 3 of the sample between the voltage probes 5 and 6. Probes 8 and 9 are connected to a meter 10 for measuring the current passing therebetween.
Note that in the drawing, the layer thickness is substantially exaggerated. In practice, the layer thickness will be significantly less than the spacing between probes 8 and 9. In this situation, it is assumed that the current passes through the entire surface layer 3 of the sample.
In the illustrated apparatus, meter 10 is designed to measure resistivity. Because the current is passing through the whole layer, the resistivity measured is equivalent to the sheet resistance R.sub.s the layer. Unfortunately, sheet resistance does not give unambiguous information concerning the specific characteristics of the material since it is also dependent on layer thickness. In contrast, to find out quantitative information about a particular metallic film's composition or stoichiometry, it is necessary to know its bulk resistivity, rather than its sheet resistance.
Bulk resistivity is related to sheet resistance by the following equation: EQU R.sub.s =.rho./t (1)
where .rho. is the bulk resistivity and t is the thickness of the thin film or layer. As can be seen by equation (1), if the sheet resistance is measured, and the thickness is known, the bulk resistivity (and thus the electrical conductivity and material composition) can be readily calculated.
In some manufacturing situations the thickness of the layer of interest is not accurately known. This often occurs in deposition processes used in microelectronic manufacturing techniques. Often times, the composition or stoichiometry of the layer of interest is known such that its bulk resistivity can be inferred. In this case (and referring to equation (1)), if the sheet resistance of the layer can be measured, the thickness (t) of the layer can then be calculated.
From equation (1) and the foregoing discussion, it should be apparent that if the sheet resistance (R.sub.s) can be measured, and one of the two remaining parameters, namely, thickness (t) or bulk resistivity (.rho.) is known, the remaining parameter can be determined. However, if neither the thickness of the layer or its bulk resistivity are known, there is presently no way of deriving either parameter independently from a measurement of only the sheet resistance. The latter situation is unfortunately quite common. For example, in some deposition techniques, an uneven layer will be deposited on a substrate. In addition, because the composition of the deposited material is not constant, the bulk resistivity at any given location is also unknown.
A similar situation is encountered in the analysis of dielectric films. Almost all measurements on such films, such as optical measurements, provide information that is also a function of both film thickness and film composition. Thus, it is not generally possible to obtain independent information on either the film thickness or the film composition from such measurements.
Layer thickness or compositional variables of both metals and dielectrics can also be analyzed through an evaluation of the thermal parameters of the material. Recently, there has been much effort devoted to developing systems for analyzing the thermal characteristics of a sample through the detection of thermal waves.
In a thermal wave detection system, an intensity modulated heating source is focused and scanned across the surface of the sample. As the beam scans across the sample, energy is absorbed by the sample at or near its surface and a periodic surface heating occurs at the modulation frequency of the heat source. This periodic surface heating is the source of thermal waves that propagate from the heated region. The thermal waves interact with thermal boundaries and barriers in a manner that is mathematically equivalent to scattering and reflection of conventional propagating waves. Thus, any features on or beneath the surface of the sample that have thermal characteristics different from their surroundings will reflect and scatter thermal waves and thus become visible to these thermal waves. Thermal waves, however, are critically damped and travel only about one thermal wavelength, thereby having a limited penetration range. In addition, due to their short length of travel, thermal waves themselves are difficult to detect. In addition, in order to detect micron size features in the sample, or to make measurements on very thin films, modulation frequencies on the order of 0.1-20 MHz are often utilized. Detection of such high frequency signals adds complexity to the detection equipment. Nevertheless, a number of systems have been developed for detecting the thermal waves.
One such detection system is disclosed in U.S. Pat. No. 4,255,971, issued Mar. 17, 1981 assigned to the same assignee as the subject invention and incorporated herein by reference. The latter patent discloses a detection technique which is based on the measurement of acoustic waves and is called thermoacoustic microscopy. When the thermal waves have been generated in a sample, a portion of their energy is always transmitted to an acoustic wave at the same frequency because of local stress and strains set up by the thermal waves. The acoustic waves are propagating waves, with much longer wavelengths than the thermal waves. They travel through condensed media with ease and are readily detected with a suitable acoustic transducer placed in acoustic contact with the sample. The magnitude and phase of the acoustic waves are directly related to the interactions undergone by the thermal waves. The magnitude or phase is then measured with suitable phase-sensitive frequency-locked electronics and recorded as a function of the position of the heating source.
As can be appreciated, the above described system, utilizing a piezoelectrical crystal attached to the sample, is a "contact" measurement technique. The latter requirement is time-consuming and potentially contaminating, and is therefore not suitable for production situations encountered in the microelectronics field. Accordingly, there has been significant work carried out in developing noncontact detection techniques. One such noncontact detection technique is described in copending applications, Ser. No. 401,511, filed July 26, 1982 and now U.S. Pat. No. 4,521,118, issued June 4, 1985; and Ser. No. 481,275, filed Apr. 1, 1983 and now U.S. Pat. No. 4,522,510, issued June 11, 1985, and incorporated by reference.
The latter applications describe a method and apparatus for detecting thermal waves by monitoring the local angular changes occurring at the surface of the sample. More specifically, when thermal waves are generated in a localized region of the sample, the surface of the sample undergoes periodic angular changes within the periodically heated area because of local thermoelastic effects. These angular changes occur at a frequency equal to the frequency of the modulated heating.
To monitor these changes, a beam of energy, such as a laser beam, is focused on the surface of the sample at a distance from the center of the heated region in a manner such that it is reflected from the sample surface. Because of the local angular changes occurring at the surface of the sample, the reflected beam will experience angular displacements in a periodic fashion. By measuring the angular displacements, information about the thermal wave activity in the sample can be determined. The latter technique has proved to be a highly sensitive process for detecting thermal waves.
An analysis of the thermal waves present in a sample can be used to derive information regarding either the thickness of thin film layers or their thermal characteristics. A complete discussion of the type of evaluation techniques which may be performed utilizing thermal waves analysis is set forth in U.S. application Ser. No. 389,623, filed June 18, 1982, and now U.S. Pat. No. 4,513,384, issued Apr. 23, 1985. As set forth therein, where the thermal parameters of a layer of interest are known, a thermal wave analysis will provide information as to the thickness of that layer. Similarly, where the thickness of a layer is known, information concerning the thermal characteristics may be derived. The latter application also discloses a technique for assigning hypothetical thicknesses to the surface of a sample in order to perform depth profiling of thermal characteristics. However, it should be clear that the limitations inherent in such a thermal wave analysis of thin films are analogous to the limitations in electrical analysis of metallic films or an optical analysis of dielectric films. More specifically, either the thickness of a layer must be known to determine the character and identity of the material defining the layer or the material must be known in order to calculate the thickness of the layer. Clearly, it would be desirable if a system was available which could provide information on both the thickness and the material characteristics of the layer or thin film being tested.
Accordingly, it is an object of the subject invention to provide a new and improved method and apparatus which permits the evaluation of both thickness and compositional variables in a layered or thin film sample.
It is another object of the subject invention to provide a new and improved method and apparatus which permits the evaluation of both thickness and compositional variables in a layered or thin film sample through the use of two independent thermal wave detection systems.
It is still another object of the subject invention to provide a new and improved method and apparatus wherein temperature variations at the surface and within the sample are detected by two independent techniques permitting the evaluation of both thickness and compositional variables in a layered or thin film sample.
It is still a further object of the subject invention to provide a new and improved apparatus wherein two independent measurements of thermal wave activity may be obtained, and wherein the detection systems share the same hardware.